This invention relates to a method for liquid supply and an apparatus therefor comprising a pressure tank and a plurality of pumps.
Frequent starting of a pump, in extreme cases, can cause overheating or seizure of the driving motor. To avoid the possibility, it has been proposed in Japanese Patent Publication No. 29403/73 to provide a plurality of pumps and operate them by turns to decrease the starting frequency of each pump. According to the prior invention, two pumps, for example, are operated for alternate periods. Each pump therefore must have sufficient capacity to meet the demand of maximum quantity of liquid supply. If an additional pump is made available when the demand is too large for a single pump, as taught by Japanese Utility Model Publication No. 31521/75, the two pumps will be able to cooperate to satisfy the demand, with less capacity each. Further, if the pump that operates first is shifted to the standby pump and vice versa whenever the preceding pump stops, then the numbers of runs of the two pumps will be equalized. Such a proposal has been made in Japanese Utility Model Publication No. 6881/77 for applications such as a pumping station using a couple of submerged pumps for draining spring water from a construction site. The apparatus disclosed operates the both pumps simultaneously or either pump alone depending on the level of spring water.
Also known in the art is a technique described in Japanese Patent Laid-Open No. 100306/76 and which employs a liquid supply apparatus having a pressure tank which permits a running pump to continue operation for some time after the pressure or liquid quantity in the tank has been again built up following the delivery, so that the frequency of pump starting may be decreased.
Those prior art teachings might be combined to provide an apparatus which includes a plurality of pumps, say, a preceding or regular pump and a standby pump, so that when the demand surpassed the capacity of the regular pump the standby pump would work, too, and, once started, the both pumps would go on running for a predetermined period of time, even after the recovery of pressure or liquid quantity to the original level within the pressure tank. It would be further possible to arrange so that the pump that operated first serves as the standby and vice versa in the next run and so forth in the ensuing runs.
However, such a combination would nevertheless be undesirable because of wasteful consumption of power. Even if the demand for liquid dwindled to a degree such that the assistance by the standby pump was no longer necessary, the pump once started would have to continue running for the predetermined period. This means that the preceding and standby pumps would both be required to continue working at the sacrifice of efficiency and economy.